Research teams in Vienna and Beijing have independently built the first working nuclear clocks, converting a long-discussed concept into operating laboratory systems. The two studies, published in Nature on October 7, use the unusually low-energy transition of the thorium-229 atomic nucleus as a timekeeping reference. Both clocks operate, but neither is yet more stable than the best optical atomic clocks.
A nuclear clock does not rely on radioactive power generation or nuclear fission. Like an atomic clock, it measures the frequency of a highly repeatable quantum transition. The difference is that conventional atomic clocks use transitions in electrons surrounding the nucleus, while the new devices use a transition inside the nucleus itself. The teams illuminated thorium-229 embedded in calcium fluoride crystals with vacuum-ultraviolet laser light near 148 nanometres and locked a laser frequency to the nuclear response.
Two independent routes to the same goal
The Vienna-led collaboration reported an optical nuclear clock with a feedback loop that stabilizes the laser against the thorium transition. Its preprint describes an instability of about three parts in a trillion divided by the square root of the averaging time, approaching the 10-to-the-minus-15 range after roughly one day. The Beijing team reported a synchronized thorium-229 clock with a comparable short-term instability of about two parts in a trillion divided by the square root of time.
The experiments used different crystal samples and technical arrangements, which makes their agreement important. Nature’s research summary said both teams showed that the frequency drifts only slowly, a necessary quality for a practical clock. The Beijing work also compared separate thorium-doped crystals and found agreement between their measured transition frequencies, supporting the idea that solid-state nuclear clocks could be reproduced across devices.
Promising, but not a replacement yet
The achievement is a proof of operation, not evidence that nuclear clocks have already surpassed today’s best timekeepers. Reuters and Nature both emphasized that the new systems remain less stable than leading atomic clocks. Optical atomic clocks can reach extraordinary precision after years of development, while the thorium systems are at the beginning of that engineering process. Laser noise, crystal environments and measurement efficiency still limit performance.
Nuclear transitions are attractive because the nucleus is more shielded from external electric and magnetic disturbances than orbiting electrons. In principle, that could make a mature nuclear clock compact, robust and sensitive to different aspects of fundamental physics. Researchers have proposed applications in navigation, network synchronization, geodesy and tests of whether the constants of nature change over time.
The solid-state approach also allows many thorium nuclei to be measured inside one crystal, potentially strengthening the combined signal and helping future devices become smaller. Crystal defects, temperature and local fields can also shift the transition, however. Comparing different crystals and tracking long-term stability will therefore be central to the next phase of work.
A new instrument for fundamental physics
The Vienna team also used its system to search for certain forms of ultralight dark matter, which could cause tiny periodic changes in the thorium transition. The experiment did not detect dark matter, but it placed new constraints on possible interactions. That result illustrates how a nuclear clock can function not only as a timekeeper but also as a detector for subtle changes in physical laws.
The next challenge is sustained improvement. Researchers will need narrower and more stable lasers, better-controlled crystals, stronger signals and repeated comparisons between independent clocks. The parallel demonstrations in Europe and China provide an important cross-check: the same nuclear transition can now be used in closed-loop clock operation by separate groups. The field has moved from spectroscopy toward actual timekeeping, while the comparison with established atomic clocks remains a target for future work.
